PROJECT SUMMARY/ABSTRACT Small cell lung cancer (SCLC), is a high-grade pulmonary neuroendocrine tumor that accounts for ~16% of all lung cancer cases diagnosed annually in the United States. In most patients, SCLC is metastatic at the time of presentation; patients with extensive stage disease have a poor prognosis with survival measureable in months, and an average 5-year survival of <5%. New therapeutic strategies are desperately needed to improve clinical outcomes for these patients. A novel antibody drug conjugate (rovalpituzumab tesirine; Rova-T) targeting Delta like ligand 3 (DLL3), a highly tumor-selective cell surface protein, has demonstrated impressive clinical benefit in early phase clinicial trials in patients with extensive stage small cell lung cancer. Among evaluable patients with DLL3+ baseline samples, as measured by IHC, a 39% overall response rate and 75% clinical benefit rate were observed, highlighting both the therapeutic potential for targeting DLL3 and the utility of DLL3 assessment. While DLL3 IHC has demonstrated diagnostic utility, this data is presently collected from archived tissue specimens, which may not serve as ideal reference points to help make the best decisions in the clinic at the time of treatment; indeed, many patients with apparent DLL3 positivity failed to respond and many patients were not evaluable. A more reliable, dynamic, real time, non-invasive yet quantitative method to evaluate the in vivo status of antigen expression on tumors will greatly improve patient selection for this agent in the clinic. We propose development of an immuno-PET diagnostic agent comprising 89Zr labeled rovalpituzumab. Specific Aim 1 builds upon promising preliminary imaging data. We are already able to obtain high-contrast immune-PET images using non-specific amine labeling and site-specific maleimide bioconjugation. We will improve upon this approach by developing more stable thiol-clickable methylsuflone chelators for 89Zr to minimize kidney uptake. Specific Aim 2 will be centered on the study of the in vivo toxicology and pharmacology of 89Zr-Rova as well as the preparation and submission of an FDA Investigational New Drug application for the clinical trial. The goal of Specific Aim 3 will be the first-in-human clinical trial of 89Zr-Rova for the PET imaging of patients with small cell lung cancer concurrently enrolled on a clinical trial of the therapeutic ADC Rova-T. This 30-patient trial will be focused on the clinical safety and efficacy of 89Zr as a predictor of response to Rova-T. This proposal will strengthen the already close working relationship between Memorial Sloan Kettering scientists and clinicians and Stemcentrx, Inc. toward the development of 89Zr-labeled rovalpituzumab. This novel imaging agent will render diagnostic value to the DLL3-targeting ADC by allowing it to serve as a contemporaneous diagnostic tool and act as a scout for future radiotherapeutics.